1. Field of the Invention
The present invention relates to a method and apparatus for filtering and drying a product. In a preferred embodiment, the present invention relates to a method and apparatus for filtering and freeze drying a frozen suspension in order to make a freeze dried powder suitable for use as a medicament.
2. Related Art
It is often desirable to filter and dry a product as part of a manufacturing process. For example, in the pharmaceutical field, it is often desirable to filter and freeze dry a frozen suspension of a drug into a dry, inhalable powder or a dry, friable powder. For example, the well known powder production process (PPP)—described in U.S. Pat. No. 6,284,283, the entirety of which is incorporated herein by reference—yields a suspension of atomized frozen protein particles suspended in liquid nitrogen (LN2) that must be filtered and dried to form a dry, inhalable powder.
In one known method for filtering and drying a drug suspended in liquid nitrogen, the suspension is transferred into porous polyester bags, wherein the liquid nitrogen is evaporated and filtered, leaving only the drug particles. The drug particles are then freeze dried by placing the bags inside a drying chamber of a conventional shelf freeze dryer (also known as a lyophilizer). In the drying chamber of a conventional shelf freeze dryer, the bags rest upon shelves that are able to be heated or cooled in order to facilitate the freeze drying. When the chamber door is closed, a vacuum pump can be actuated to reduce the pressure in the chamber, which freeze dries the product by causing the moisture to sublime. While the pressure is being reduced, the shelves are heated, which facilitates drying of the product. After drying is complete, the vacuum is relieved and the door to the chamber opened. The bags are removed from the chamber and the freeze dried drug powder is harvested from the bags by inverting each bag over another container.
This known method and apparatus for filtering and drying a product suffers from several drawbacks. First, the product must be filtered and freeze dried in separate containers, necessitating transfer of the product to the freeze drier in order to dry it. In addition, the freeze dryer requires the use of secondary containers (i.e., bags) inside of the drying chamber. Also, in the shelf freeze drier, the heat source (i.e., heat transfer fluid inside the shelf) is not located a uniform distance from the product, which may result in uneven or incomplete freeze drying. Further, the transfer of the product from bags to a jar can result in product loss.
Other known freeze dryers also suffer from drawbacks. For example, U.S. Pat. No. 5,884,414 discloses a freeze dryer in which the product to be dried is placed directly into the drying chamber. However, this dryer does not have the ability to filter the product and does not have a heat transfer source for heating or cooling the product. U.S. Pat. No. 3,731,392 discloses a freeze dryer, in which the product to be dried is freeze dried in multiple, continuous drying chambers. However, this dryer does not have the ability to filter the product.
Thus, there is a need in the art for an improved method and apparatus for filtering and drying a product. What is needed is a method and apparatus that allows a product to be filtered and dried in a single container, without necessitating the use of secondary containers. What is also needed is a method and apparatus in which the heat transfer source and the vacuum source remain close to the product, to better penetrate a product for more uniform and efficient drying. In addition, what is needed is a method and apparatus that allows for direct harvesting of the product from the container holding the product to avoid spillage and waste of product. Such an apparatus also would have a compact shape and design to be convenient for use. The present invention, which is fully set forth below, fulfills these and other needs for an improved method and apparatus for filtering and drying a product.